I read that a 16GB iPhone 5S costs $198.70 to produce. Of course that price does not include R&D and other costs that go into producing something. You may think the price is a rip-off, but the free market lets Apple charge that price and people line up to pay it.
So with your reasoning I would expect Apple to give you only $200 to $250 on release day for your used iPhone 6. Thanks for clearing that up. Makes perfect sense now.